As is more particularly illustrated by FR-A-2,333,200, a steam generator equipping a nuclear power station conventionally comprises a vertically axed, outer envelope, whose internal space is subdivided into two portions in the heightwise direction by a horizontal plate known as a tube plate. The ends of the tubes of an inverted U-shaped tube bundle are fixed to the tube plate and issue below the latter respectively into an admission collector and into a discharge collector for the water circulating in the primary circuit of the reactor, known as primary water. The water circulating in the secondary circuit of the reactor, known as secondary water or feed water, is injected into the part of the steam generator positioned above the tube plate by a torodial or semitorodial main supply collector. This supply collector is conventionally placed above an annular recirculation space formed between the outer envelope and an inner envelope covering the tube bundle and whose lower edge is spaced from the tube plate.
In steam generators of this type, the installation of the secondary water supply collector takes place above the annular recirculation space, but below the level of the water contained in the outer envelope. However, in the absence of special precautions, a stoppage or reduction in the flow of water in the reactor secondary circuit could lead to an emptying of the supply collector causing pressure jumps and surges during the restarting of the pumps installed in the secondary circuit.
As illustrated by FR-A-2,333,200, this problem has been solved by equipping the supply collector with inverted J-shaped tubes by which the collector issues above the annular recirculation space. However, this procedure suffers from the disadvantage of making the manufacture of the steam generator more complicated and therefore increasing the length of the manufacturing process.
Moreover and as is more particularly illustrated by U.S. Pat. No. 3,804,069, 3,896,770 and 3,916,843, consideration has also been given to supplying the secondary water to the steam generator by directly connecting a secondary water admission tube to a lower part of the inner envelope, so as to cause the secondary water to enter directly at the base of cold branches of the tubes of the bundle. More specifically, deflectors positioned facing the admission tube and around the cold branches then form a device for preheating the secondary water flowing between the tubes.
If the aforementioned solution makes it possible to eliminate any pressure jump or surge risk following a stoppage of the water flow in the secondary circuit, it still has the disadvantage of subjecting the lower parts of the cold branches of tubes to significant transverse flows, while preventing controlled distribution of the secondary water flow over the steam generator cross-section.
Moreover, if the migrating bodies or members such as filler rods, screws, bolts, etc., inadvertently introduced into the secondary circuits during manufacture enter the steam generator through the secondary water admission tube, they can become wedged between and therefore damage the tubes of the bundle.